1. Field of the Invention
The present invention relates to voltage regulators. More particularly, the present invention relates to adjusting the on-time for a discontinuous switching voltage regulator.
2. Description of the Prior Art
Discontinuous switching voltage regulators comprise a charging element that is charged/discharged and then tristated over a cycle that begins when an error signal exceeds a threshold. For example, the charging time may begin when an output voltage falls below a reference voltage, or when a propagation oscillation signal representing a propagation delay of a digital circuit falls below a reference oscillation signal. In a discontinuous buck mode switching voltage regulator, the charging element typically comprises an inductor that is connected to a source voltage through an upper switch (e.g., a field effect transistor or FET) while charging during an “on-time,” and connected to ground through a lower switch (e.g., using a diode or FET) while discharging during a “discharge time.” Once the inductor discharges, both the upper and lower switches are opened to tristate the inductor during a “tristate time” in order to prevent an output capacitor from discharging through the inductor to ground. The inductor remains in tristate until the error signal exceeds the threshold which starts a new cycle.
The on-time of the discontinuous mode cycle may be established relative to the current flowing through the inductor such that the upper switch is turned off when the current exceeds a peak current threshold. The prior art has suggested to adjust the peak current threshold according to a linear transfer function of the estimated load current to improve power efficiency and reduce ripple voltage. However, employing a linear transfer function to adjust the peak current threshold may decrease power efficiency due to an increase in the transient response time, and it may also lead to instability. In addition, employing an analog current sensor and a programmable analog current comparator can increase the cost and complexity of the switching voltage regulator.
There is, therefore, a need to adjust the on-time of a discontinuous switching voltage regulator to improve power efficiency while maintaining an acceptable transient response. There is also a need to avoid the cost and complexity of a current controlled loop using an analog current sensor and a programmable analog current comparator.